Method of manufacture of top coated cellulosic panel

ABSTRACT

A coated panel formed from a self-supporting cellulosic substrate and a top coating containing a thermoset resin having about 0.1-1.5 wt % free formaldehyde admixed with said resin; and (b) a thermoplastic polymer thoroughly admixed with said resin, wherein said thermoplastic polymer exhibits amine groups capable of reacting with at least some of the free formaldehyde under resin curing conditions. The coated panel can be embossed with decorative patterns such as wood grain without fracture of the coated surface or significant buildup on the press or embossing die surfaces.

FIELD OF THE INVENTION

The present invention relates to cellulosic panels exhibiting a topcoating receptive to embossing with a desired pattern such as a woodgrain.

BACKGROUND OF THE TECHNOLOGY

The construction of houses and other buildings involves the use of avariety of materials for walls, floorings and other surfaces. Solidhardwood or soft wood boards are highly desired for such surfaces, butsolid boards are often prohibitively expensive. Veneer panels have oftenbeen used as an alternative for wall surfaces, but such panels posetheir own concerns. As trees of the required type, size and qualitybecome more and more scarce, the manufacture of multilayer veneers orplywoods is expensive with high quality veneer panels becoming difficultto obtain.

Gypsum boards or similar substrates are in widespread use as analternative to solid boards or veneers. These synthetic panel stockmaterials are typically made from two outer layers of a thick papermaterial having an inorganic material, e.g., gypsum or calcium sulphate,in between. Gypsum board suffers from substantial loss of strengthand/or structural integrity if the board becomes wet. Moreover, gypsumboards have no inherent grain structure so there is little inherentretention strength for nails, screws or the like which might be used forhanging paintings, photographs, ornaments, or shelving.

The competing needs of reasonable construction costs with high qualitybuildings has led to expanded uses for alternative wood products. Forinstance, particle board, fiber board, oriented strand board (OSB),hardboard, and other similar boards are formed from wood that may nototherwise be usable in the construction industry. Boards are also formedfrom particles, chips, flakes or other fragments of wood. These boardstock are being used more and more in the construction of buildings,particularly for wall and floor surfaces and sub-surfaces. Such boardshave a quality and integrity that is more than adequate for such uses.

Some of these alternative boards are vulnerable to swelling when exposedto moisture or water. These boards have been coated with wax orotherwise treated to avoid the problems with water. Lund U.S. Pat. No.4,241,133 discloses that wood flakes may be bonded together with abinder. Examples of the binders include urea/formaldehyde resins,phenol/formaldehyde resins, melamine/formaldehyde resins andpolyisocyanates. Binder concentrations of between 5 and 12% aredisclosed. Waxes may be used for water resistance and preservatives mayalso be added. Other methods of manufacture of particle and similarboards are disclosed in U.S. Pat. Nos. 3,164,511 to A. Elmendorf;3,391,233 to B. Polovtseff; and 3,940,230 to E. Potter.

Aminoplast resins like melamine-urea-formaldehyde (MUF) resins are usedas a top spray on resin-containing wood fibers just before pressing thefibers into medium density hardboard. As the binder resin cures underheat and pressure, the board is provided with its structural properties.Simultaneously, the top spray resin cures and seals the surface with ahard protective coating. The following references disclose methods ofpreparing the thermoset resins used as the "The Chemistry of SyntheticResins" by Carleton Ellis, Reinhold Publishing Co., 1935; "PhenolicResin Chemistry" by N. J. L. Megson, Academic Press Inc., New York,1958; "Aminoplasts" by C. P. Vale, Cleaver-Hume Press, Ltd., London,England; and British Pat. No. 480,316.

Often, a wood grain is molded into the board surface during the pressingstep. There are times, however, when it is desirable to emboss a woodgrain or other pattern into the surface of a finished board containing atop spray, such as MUF. See, Book U.S. Pat. No. 4,266,925 which isherein incorporated by reference. The embossing process involves theapplication of heat and pressure to the surface of the board whichfractures the hard, brittle MUF coating. The resulting surface isunacceptable as well as weakening the cellulosic panel and rendering thesurface vulnerable to humidity. Water extractable lignins will migrateto the surface through the fractures thereby causing surfacediscoloration and yellowing.

The present invention is directed to the embossing of cellulosic panelshaving a top spray coating that is otherwise too brittle to embosswithout significant fracture. Specifically, the invention addresses atop spray coating and the use thereof in an embossing process that doesnot fracture the coating.

The brittleness of top spray coatings has been the subject of someconcern in the art. Melamine-formaldehyde, urea-formaldehyde, andmelamine-urea-formaldehyde polymers are often modified with glycols,sugars, and various latexes in attempts to reduce the brittleness of thethermosetting resins. Some attempts have been successful but at the costof using modifying materials which may volatilize at embossingtemperatures or otherwise migrate from the top coat to leave an uncured,low molecular weight residue on the surface of the press or embossingdie surfaces. This buildup results in frequent nonproductive maintenancetime for cleaning.

It would be useful to have a top coating that would be sufficientlyflexible to accept embossing without materially affecting the hardthermosetting properties of the final coating or causing buildup on thepress or embossing die surfaces.

The art has also investigated the embossing of panels having a basecoatfinish on top of the formed panel. In these methods, a thermoplastic ororganic solvent-based thermosetting basecoat is generally applied to thepanel before embossing. Unfortunately, the conventional basecoats softenat press temperatures that are high enough to get good embossing atreasonable pressures, typically over 300° C. and 5-9 Mpa. Pieces of thebasecoat separate and stick to the embossing die surface when disengagedfrom the panel surface. The resulting product panel thus exhibits asurface having irregular areas lacking a basecoat and an interruptedfinish. Such panels are unsuitable for further finishing.

Films have been used in attempts to prevent embossed basecoatings fromsticking and separating. A thin film of a heat resistant material havinga thickness of 0.5 to 1.5 thousandths of an inch, (e.g., Mylar™ film) ispositioned between the embossing die and the board having the basecoat.Films can be effective at eliminating separation of the print basecoatand (depending on its thickness) does permit at least some limitedamount of fine line wood grain detail to be retained. Films require,however, an extra handling step in the manufacturing process and cannotbe reused. Such extra costs are often not worth the limited degree ofdetail obtained.

Resin-impregnated paper overlays are also known as protective media thatmight be used to accept embossed details. Such overlays come in mediumand high densities and are basically made of paper containing partiallycured phenol-formaldehyde resin. The paper overlays are bonded to awooden panel using heat and pressure. Unfortunately and in addition tothe handling costs for sheets, the exposed surface of the overlay isalso sufficiently reactive that a release agent is required to preventthe resin from sticking to the embossing dies.

It would be desirable to have a method for embossing cellulosic panelsthat does not foul the press or embossing die surfaces or require theuse of either separator sheets or embossable resin-impregnated sheets.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a top spray composition thatwill not adversely affect the hard thermosetting properties ofconventional top spray resins while exhibiting flexible thermoplasticproperties upon reheating sufficient to accept texture embossing withlittle or no buildup on press or embossing die surfaces.

It is another object of the invention to provide a panel and method formanufacture thereof that uses the top spray coating which does notfracture the board surface during the embossing process.

In accordance with these objectives and others that will become apparentfrom the description herein, the present invention provides a top spraycoating composition exhibiting a softening point within the range fromabout 130° to about 300° C. wherein said top coating comprises: (a) athermoset resin having free formaldehyde admixed with said resin in anamount within the range from about 0.1 wt % to about 1.5 wt % based onsaid thermoset resin; and (b) a thermoplastic polymer thoroughly admixedwith said resin, wherein said thermoplastic polymer exhibits aminegroups capable reacting with either the amine reactive alkylol groups onthe resin or free formaldehyde admixed with said resin under resincuring conditions.

The invention also provides a cellulosic panel coated with theabove-described composition as well as a method of embossing cellulosicpanels that have been treated with the top coating composition.

The invention provides a cellulosic panel with the flexibility to impartan embossed pattern that will accept fine line detail without fracturingthe panel surface. While not wishing to be bound by theory, it appearsthat the mixture of a thermoplastic resin with a thermoset resin doesnot affect the hard thermosetting properties while exhibiting someflexible thermoplastic properties upon reheating with little or noembossing plate buildup.

DESCRIPTION OF THE INVENTION

The present invention relates to a panel formed from a cellulosicsubstrate with a hard, polymeric top spray coating that will becomesufficiently flexible upon heating to embossing temperatures to acceptembossed details without significant fracture of the surface. Uponcooling, the top coat will reharden thereby retaining even fine embosseddetails without fracture of the panel surface. The result is acellulosic board product that can be readily embossed for use indecorative applications without the previous loss of structuralintegrity or vulnerability to moisture.

The planar panel used to make embossable stock in the invention is ofconventional manufacture being made or derived from resin-boundparticles, chips, flakes, sawdust, paper and/or other fragments of hardor soft woods in quantities sufficient to produce a self-supporting,planar wood product. Examples of trees that will serve as a source ofsuch cellulosic materials include, but are not limited to, aspen, beech,birch, cedar, Douglas and other firs, hemlock, pine and spruce in theU.S. and Canada.

The substrate materials used in the present invention are generallypressed into shape from wood fragments coated with a bonding resin.Examples of suitable bonding resins include, inter alia,urea/formaldehyde resin, phenol/formaldehyde resin,melamine/formaldehyde resin, polymeric isocyanate resin and the like.The bonding agent is generally a liquid rather than a powder and ispreferably a phenol/formaldehyde resin. Bonding resins are typicallyused in an amount within the range from about 1.8 to about 2.3 wt %based on the wood fragments.

A wax (e.g., a petroleum wax), may also be applied to the wood fragmentsat an amount within the range from about 1-2 wt % based on the woodfragments to improve water resistant properties. Conventionalpreservatives and other additives may also be used if desired.

The key to the present invention is the hard, top spray coatingcontaining a thermoset resin in admixture with a thermoplastic resin ina quantity sufficient to exhibit a softening temperature within therange from about 130° to about 300° C. This minimum softeningtemperature is often encountered during conventional embossing processesyet is sufficiently high that normal uses for embossed panels madeaccording to the invention will not soften and lose their embosseddetail. Moreover, the softening is sufficient that there is nosignificant fracturing of the panel surface during the embossing andexhibits little or no transfer of material to the press or embossing diesurfaces.

The quantity of top spray coating that is applied to the cellulosicsubstrate can vary depending on the quality and nature of the cellulosicmaterial used. In general, top spray coating is applied to theresin-coated cellulosic material from a 25% aqueous solution at a ratewithin the range from about 0.5 to about 2 grams of solid resin persquare foot of panel before the cellulosic material is pressed into apanel structure. A preferred application rate of top coat according tothe invention is within the range from about 1 to about 1.5 grams ofsolid resin per square foot of panel.

The Thermoset Resin Component

Three classes of thermoset resins are preferred in the top spray coatingaccording to the invention: phenoplasts, aminoplasts, andketone-aldehyde condensation polymers. These polymers are made from suchresins as the acid or base catalyzed phenol-aldehyde resins,urea-aldehyde resins, melamine-aldehyde resins, acetone-aldehyde resins,etc., mixtures and copolymers thereof, e.g. melamine-urea-formaldehyde(MUF) resins. The MUF resins are the preferred compositions for use asthe thermoset resin component.

Specifically, the aldehyde condensation polymers which can be used asthe thermoset resin component include (1) phenoplasts comprising thecondensation polymers of an aldehyde such as formaldehyde with aphenolic type material having at least two positions ortho and/or parato the hydroxyl group open for reaction, such as phenol,phenol-resorcinol, xylenol, cresol, resorcinol, and their derivatives,(2) aminoplasts comprising the condensation polymers of an aldehyde suchas formaldehyde with compounds such as benzoguanamine, dicyandiamide,urea, melamine-urea, melamine, and their derivatives, and (3)ketone-aldehyde condensation polymers such as acetone-formaldehyde,methyl ethyl ketone formaldehyde, methyl isobutyl ketone formaldehyde,and the like. Another useful resin is the known ortho-condensedphenolformaldehyde resin made by condensing 0.7 to 1.0 molesformaldehyde with 1 mole phenol in the presence of an ortho-directingcatalyst such as calcium acetate.

The aldehydes used in preparing the aminoplasts may be monofunctional(i.e. a monoaldehyde) or polyfunctional, having at least two aldehydegroups separated by at most one carbon atom. Examples of usefulaldehydes include, inter alia, formaldehyde, paraformaldehyde,polyoxymethylene, trioxane, acrolein, and aliphatic or cyclic aldehydessuch as glyoxal, acetaldehyde, propionaldehyde, butyraldehyde, andfurfuraldehyde.

The condensation reaction with formaldehyde, furfuraldehyde,paraformaldehyde, polyoxymethylene or trioxane can be performed in abatch or continuously. The reactants are condensed in the presence of amildly acidic or alkaline catalyst although the reaction may beconducted at slower rates without catalysts.

The condensation process with acrolein, glyoxal, acetaldehyde,propionaldehyde, or butyraldehyde is performed incrementally. Thecondensation reaction is conducted in stages by combining the reactantsin the presence of a strongly acidic catalyst, neutralizing the reactionproduct, incorporating additional aldehyde into the neutralized productfor further reaction in the presence of a mildly acidic or alkalinecatalyst.

Preferred resins are aminoplast resins with water-soluble, liquid,thermosetting phenol-aldehyde resins being the most preferred resins foruse as the thermoset component of the top coat composition according tothe invention. Novolacs, because they lack reactive alkylol groups, arenot directly useful in this invention although they may be furtherreacted with aldehyde to convert them to useful resoles.

For top coatings of the invention, the thermoset resin component (with afree formaldehyde content within the range from about 0.1 wt % to about1.5 wt % based on the thermoset resin) is admixed with a thermoplasticpolymer having reactive amine groups to give a top coat that will softenat a temperature within the range from about 130° to about 300° C.Preferably, the top coat composition softening temperature is within therange from about 130° to about 180° C. and most preferably within therange from about 130° to about 150° C. These softening pointtemperatures are within the range of board surface temperaturesencountered during a typical embossing process where the embossing diesurface may be significantly higher, e.g., 300° C. and above. The boardsurface is, therefore, imparted with detail from the die pressure whilebeing controllably darkened by the heated die surface.

Without wishing to be bound by a particular theory of operation, itappears that the thermoplastic polymer becomes linked to the thermosetresin through reaction with the free formaldehyde to impart flexibilityand provide a composition exhibiting both the hard thermosettingproperties of the thermoset resin as well as some flexible thermoplasticproperties upon reheating. Reaction through the free formaldehyde locksthe thermoplastic into the cured polymer thereby avoiding the release ofuncured resins that would result in a buildup on the press and/orembossing die surfaces.

The Thermoplastic Component

The polyamide and aminopolyamide compositions useful in the presentinvention embrace those semi-crystalline and amorphous resins having amolecular weight of at least 5000 having a linear or branched structure.The phrase "polyamide" refers to a condensation product that containsrecurring aromatic and/or aliphatic amide groups as integral parts ofthe main polymer chain, such products being known generically as"nylons". Preferably, these polyamides have molecular weights of fromabout 5,000 to about 50,000. Furthermore, the polyamides are preferablylinear with a melting point in excess of 200° C. These polyamides may beα-polyamides, α,ω-polyamides, and mixture and/or copolymers of these.

By "α-polyamides" is meant those polyamides having only one terminalgroup which is reactive with formaldehyde. Amine groups are preferredreactive groups. Examples of suitable α-polyamides may be obtained bypolymerizing a monoaminocarboxylic acid or an internal lactam thereofhaving at least two carbon atoms between the amino and carboxylic acidgroups thereof. Suitable polyamides include those described in U.S. Pat.Nos. 2,071,250; 2,071,251; 2,241,322; and 2,312,966, the disclosures ofwhich are herein incorporated by reference.

As examples of monoaminocarboxylic acids or lactams monoaminocarboxylicacids, there may be mentioned those compounds containing from 2 to 16carbon atoms between the amino and carboxylic acid groups, said carbonatoms forming a ring with the --CO--NH-- group in the case of a lactam.As particular examples of amino-carboxylic acids and lactams there maybe mentioned ε-aminocaproic acid, butyrolactam, pivalolactam,caprolactam, capryllactam, enantholactam, undecanolactam, dodecanolactamand 3- and 4-amino benzoic acids.

Illustrative examples of α-polyamides which may constitute in whole orin part the thermoplastic polymer component include: polypyrrolidone(nylon 4); polycaprolactam (nylon 6); polyheptolactam (nylon 7);polycapryllactam (nylon 8); polynonanolactam (nylon 9);polyundecanolactam (nylon 11); and polydodecanolactam (nylon 12).

It is also possible to use in this invention polyamides prepared by thecopolymerization of two or more of the above polymers orterpolymerization of the above polymers or their components.

By "α,ω-polyamides" is meant those polyamides having at least twoterminal amine groups, e.g. on each end of a linear polyamide, which arereactive with formaldehyde and/or the amine or alkylol groups of thethermoset resin component.

Examples of such α,ω-polyamides are those polyamides that may beobtained by polymerizing a diamine which contains at least two carbonatoms between the amino groups thereof and a dicarboxylic acid or esterthereof. Suitable α,ω-polyamides include those described in U.S. Pat.Nos. 2,071,250; 2,071,251; 2,130,523; 2,130,948; and 3,393,210, thedisclosures of which are herein incorporated by reference. Typically,these polyamides are prepared by polymerizing substantially equimolarproportions of the diamine and the dicarboxylic acid. Excess diamine maybe employed to provide an excess of amine end groups over carboxyl endgroups in the polyamide or, vice-versa, to provide an excess of carboxylend groups over amine end groups in the polyamide.

The term "substantially equimolar proportions" in reference to thediamine and dicarboxylic acid reactants is used to cover both strictequimolar proportions and the slight departures therefrom which areinvolved in conventional techniques for stabilizing the viscosity of theresultant polyamides.

Examples of these diamines have the general formula H₂ N(CH₂)_(n) NH₂wherein n in an integer of from 2 to 16, such as trimethylenediamine,tetramethylenediamine, pentamethyldiamine, octamethylenediamine,decamethylenediamine, dodecamethylenediamine, hexadecamethylenediamine,and hexamethylenediamine.

Other examples of suitable diamines include C-alkylated diamines (e.g.,α,2-dimethylpentamethylenediamine and 2,2,4-trimethylhexa-methylenediamine), aromatic diamines (e.g., p-phenylenediamine,4,4'-diaminodiphenyhlsulphone, 4,4 '-diaminodiphenyl ether and4,4'-diaminodiphenyl sulphone, 4,4'-diaminodiphenyl ether and4,4'-diaminodiphenylmethane), and cycloaliphatic diamines likediaminodicyclohexylmethane.

Suitable dicarboxylic acids may be aromatic noting isophthalic andterephthalic acids as examples. Preferred dicarboxylic acids are of theformula HOOC--Y--COOH wherein Y represents a divalent aliphatic radicalcontaining at least 2 carbon atoms, and examples of such acids aresebacic acid, octadecanedioic acid, suberic acid, azelaic acid,undecanedioic acid, glutaric acid, pimelic acid, and adipic acid. Oxalicacid may also be used. Furthermore, the dicarboxylic acid may be used inthe form of a functional derivative thereof, for example an ester.

Illustrative examples of α,ω-polyamides which may be used in themodifying the condensation polymer in the top coat composition of theinvention include: polyhexamethylene adipamide (nylon 6:6);polyhexamethylene azelaiamide (nylon 6:9); polyhexamethylene sebacamide(nylon 6:10); polyhexamethylene isophthalamide (nylon 6:IP); polyamideof hexamethylenediamine and n-dodecanedioic acid (nylon 6:12); andpolyamide of dodecamethylenediamine and n-dodecanedioic acid (nylon12:12). Preferred α,ω-polyamides include 6,6; 6,3; and 6,12. Polyamidesprepared by the copolymerization of two or more of the above polymers orterpolymerization of the above polymers or their components.

Also useful is nylon produced by Dynamit Nobel, which is the product ofthe dimethyl ester of terephthalic acid and a mixture of isomerictrimethyl hexamethylenediamine.

A preferred example of a thermoplastic polymer containing reactive aminogroups is an aminopolyamide, such as those disclosed in U.S. Pat. Nos.2,926,116 to Keim and 3,951,921 to Espy et al. See, example 1 of each ofthese patents. It is not necessary, however, that the aminopolyamide bereacted with an epoxide as disclosed in those patents. Theaminopolyamide may be prepared by reacting a dicarboxylic acid with apolyalkylene polyamine under such conditions as to produce a long chainaminopolyamide that is preferably water soluble.

Suitable dicarboxylic acids that can be used to prepare theaminopolyamide include diglycolic acid and the saturated aliphaticdicarboxylic acids containing from 3 through 12 carbon atoms, such assuccinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid,azelaic acid, sebacic acid, methyl adipic acid, and methyl glutaricacid.

Other suitable dicarboxylic acids include the aromatic acids such asterephthalic acid, isophthalic acid, and phthalic acid; andalpha-beta-unsaturated dicarboxylic acids such as maleic acid, fumaricacid, itaconic acid, glutaconic acid, citraconic acid, and mesaconicacid.

The available anhydrides of the above acids can be used in preparing theaminopolyamide. The amides of the above acids can also be used toprepare the aminopolyamide. Esters of the above acids can be employed inpreparing the aminopolyamide, if desired. Another ester that can be usedis an ester of malonic acid, such as, for example, dimethyl malonate,diethyl malonate, and dipropyl malonate. Mixtures of any two or more ofthe above reactants can be used to prepare the aminopolyamides. Thus,for example, a mixture of two different acids can be used; a mixture oftwo different anhydrides can be used; a mixture of two different esterscan be used; a mixture of two different amides can be used; a mixture ofat last one acid and at least one ester can be used; a mixture of atleast one anhydride and at least one acid can be used; and a mixture ofat least one acid, at least one anhydride, and at least one ester can beused.

The polyalkylene polyamine employed can be represented by the formula

    H.sub.2 NC.sub.n H.sub.2n [N(R")C.sub.n H.sub.2n ].sub.x NH.sub.2

wherein R" is hydrogen, C₁ -C₁₂ alkyl, or C₁ -C₁₂ hydroxyalkyl; n is aninteger 2 through 6 and x is an integer 1 through 4. Examples of C₁ -C₁₂alkyl are methyl, ethyl, butyl, hexyl and dodecyl. Examples of C₁ -C₁₂hydroxyalkyl are hydroxyethyl, 2-hydroxypropyl, 2-hydroxybutyl and2-hydroxydodecyl.

Specific examples of polyalkylene polyamines of the above formula thatcan be employed include diethylenetriamine, triethylenetetramine,tetraethylene pentamine, dipropylenetriamine, dihexamethylenetriamine,pentaethylenehexamine, iminobis(propylamine), and methylbis(3-aminopropyl)amine.

Other polyalkylene polyamines that can be employed and which are notincluded in the above formula include 1,4-bis(3-aminopropyl)piperazineand 1-(2-aminoethyl)piperazine. Mixtures of two or more polyalkylenepolyamines can be used if desired.

The spacing of the amine nitrogens in the aminopolyamide can beincreased, if desired. This can be accomplished by substituting adiamine such as ethylenediamine, 1,3-diaminopropane, 1,2-diaminopropane,hexamethylenediamine, aminoethylethanolamine and the like for a portionof the polyalkylene polyamine. For this purpose, up to about 80% of thepolyalkylene polyamine can be replaced by a molecularly equivalentamount of diamine. Usually a replacement of about 50% or less will beadequate.

Temperatures employed in carrying out reaction between the dicarboxylicacid and the polyalkylene polyamine can vary from about 50° C. to about250° C. or higher at atmospheric pressure. Temperatures between about80° C. and 210° C. are preferred. Lower temperatures can be employed byusing reduced pressure. Time of reaction will be from about 1/2 hour to4 hours and will vary inversely with temperature.

In carrying out the reaction, it is preferred to use an amount ofdicarboxylic acid sufficient to react substantially completely with theprimary amine groups of the polyalkylene polyamine but insufficient toreact with the secondary amine groups and/or tertiary amine groups toany substantial extent. This will usually require a mole ratio ofpolyalkylene polyamine to dicarboxylic acid of from about 0.9:1 to about1.2:1. However, mole ratios of from about 0.8:1 to about 1.4:1 can beused.

As noted above, the preferred thermoset reins are condensation polymersthat are polar in nature. The thermoplastic polymers admixed with theseresins should also be polar for adequate chemical compatibility. Polarthermoplastic polymers are those which contain at least one polarfunctional group capable of chemically reacting with free formaldehydeand, to a lesser extent, be reactive toward alkylol groups on thethermoset resin. Examples of such functional groups on thermoplasticpolymers that will be compatible with polar condensation resins include,inter alia, amino groups, alkylol groups, hydroxyl groups, thiol groups,carboxyl groups, isocyanate groups, epoxy groups, halogen groups, andtheir derivative groups including urethane groups, ester groups, amidegroups, ammonium salt groups, and metal carboxylate groups. Thepreferred polar groups for the thermoplastic polymers are amino groupsand alkylol groups.

The polar functional groups for the thermoplastic component may bebonded to either the terminal ends, the side chains or backbone of thethermoplastic polymers. As preferred examples of the polar thermoplasticpolymer containing such polar functional group(s), examples include,inter alia, polyamides, thermoplastic polyesters, thermoplasticpolyurethanes, vinyl alcohol polymers, vinyl ester polymers, ionomersand halogenated thermoplastics. Such polar thermoplastic polymers aredisclosed in U.S. Pat. Nos. 4,080,357; 4,429,076; 4,628,072; 4,657,970;4,657,971; and 4,906,687, the disclosures of which are hereinincorporated by reference.

It should be noted that polar thermoplastic polymers having more thanone reactive functional group can result in varying and controllabledegrees of cross-linking. Such a phenomenon can be advantageously usedto control the degree of thermoplastic flexibility imparted the topcoating composition thereby permitting a reduction in the crosslinkdensity of the condensation polymer.

Options, Panel Construction and Process

The top coat composition may be formulated with a catalyst to promotereaction between the thermoset and thermoplastic components as well asreduce buildup on the press and embossing dies. A particularly preferredcatalyst is a mixture of sulfuric acid and triethylamine commerciallyavailable from Georgia-Pacific Resins, Inc. as GP 4590 (CAS No.54272-29-6).

The construction panel product of the invention may optionally also havea layer of paper or veneer interposed between the panel and the topcoat. A wide variety of papers may be used, but it is preferred that thepaper be a bleached or, preferably, unbleached kraft paper.

The paper may be used in a variety of thicknesses depending on themanufacturing process and the nature of the wood fragments used in thatprocess. Some processes may impose higher demands on strength propertiesof the paper to resist puncture during manufacture. Preferred thicknessof kraft paper are within the range from about 3 to about 6 thousandthsof an inch in thickness (75-150 microns).

The construction panels of the invention may be performed with a varietyof methods. As an example, a cellulosic panel with an optionalintervening layer of paper or veneer may be coated with the top coat toform a composite stock. The resulting composite is then heated undercuring conditions for the binder resin to effect curing of the top coatand bonding of the layers to each other.

The curing temperature is preferably above the melting point of thethermoplastic polymer to effect bonding. Preferred temperatures are inexcess of 130° C. and preferably within the range from about 150° C. toabout 200° C. to cure the binder resin as well as the top coating.Pressure is preferably applied simultaneously in a press or betweenrollers either with or without a textured surface for embossing a detailpattern in at least one of the panel surfaces. Embossing pressuresgenerally are within the range from about 700-1500 psi using either dieplates or rollers.

In an alternative method, the panels may be manufactured in a one-stepprocess. In such a process, a layer of top coat film is fed onto ascreen or other support surface. A layer of wafers or other fragments ofwood is then laid onto the top coat film, in an amount to provide apanel of the required thickness. The wafers would normally be coated oradmixed with a binder, e.g. a phenol/formaldehyde resin. A second topcoat film may then optionally be fed onto the wafers. The resultantcomposite is then fed between heated rollers (with or without a detailpattern on at least one of the surfaces thereof) at a temperature andpressure sufficient to bond the composite and form the panel. Such aone-step process may be operated in a continuous manner.

Embossing of panels according to the invention need not be performedimmediately after or concurrently with the panel formation. Pre-formedpanels can be run through embossing dies in a press heated to atemperature above the softening point of the top coat in a batch processor continuously. Embossing dies can be in the form of platens orrollers.

The construction panels of the invention may be used in a variety ofend-uses, depending in particular on the nature of the panel. Forinstance, the panels may be used as the interior surfaces of buildings.Panels may be painted to provide an attractive surface, the top coatproviding a surface that is capable of being painted while minimizingthe uptake of paint by the panel. For panels including a decorativepaper layer, the layer of paper may also be decorated in other manners.Embossed panels may be shellacked or coated with varnish or the like, topreserve and enhance the attractive features of the embossing. Thepanels may be nailed and are capable of accepting paintings and otherwall decorations. The panels may also be used as sub-layers in theconstruction industry, to provide barriers to moisture or the like andto provide a surface that is capable of having other layers adheredthereto.

The invention is conveniently described with reference to the followingexamples. It will be understood, however, that the examples should notbe construed as limitations on the scope of the appended claims.

EXAMPLES Example 1

Preparation of Thermoplastic Component

A stirred mixture of 200 parts of diethylenetriamine and 290 parts ofadipic acid is heated to 170°-175° C. for 1.5 hours with evolution ofwater, cooled to 140° C. and diluted to 50% solids with about 400 partsof water. The resulting aminopolyamide (dimethylenetriamine-adipic acidpolyamide) has a reduced specific viscosity (RSV)=0.16 (defined as nsp/Cin 1 molar aqueous NH₄ Cl at 25° C. at C=2 g./100 ml.).

Example 2

Preparation of Thermoplastic Component

Two hundred twenty-five grams (2.18 moles) of diethylenetriamine and 100grams of water were placed in a 3-necked flask equipped with amechanical stirrer, thermometer and condenser. To this was added 290grams (2.0 moles) of adipic acid. After the acid had dissolved in theamine, the solution was heated to 185°-200° C. and held there for 1 1/2hours. Then vacuum from a water pump was applied to the flask during theperiod required for the contents of the flask to cool to 140° C.following which 430 grams of H₂ O was added. The polyamide solutioncontained 52.3 % solids and had an acid number of 2.1.

Examples 3-7

Top spray coatings with the formulations shown in Table 1 are preparedfor testing to detrmine flexibility upon heating. The product qualitiesof the coating composition are listed in Table 2.

                  TABLE 1                                                         ______________________________________                                        Coating Ingredients                                                                       (% Conc)  3      4    5    6    7                                 ______________________________________                                        MUF Base Resin,                                                                           61        68.0   90.0 87.0 78.0 95.2                              GP 5361.sup.1                                                                 Catalyst, GP 4590                                                                         100       1.4    1.4  1.4  1.4  1.4                               Polyvinylalcohol                                                                          15        30.2   --   --   --   --                                Jeffamine ED2001.sup.2                                                                    100       --     8.2  --   --   --                                Polyvinylacetate                                                                          55        --     --   11.2 --   --                                Polyamide ROPL                                                                            20        --     --   --   20.2 --                                2707.sup.3                                                                    Sodium Hydroxide                                                                          25        .4     .4   .4   .4   .4                                ______________________________________                                         .sup.1 MUF Resin 5361 includes melamine (17.13%), urea (11.77%),              formaldehyde (47.74%), methanol (16.49%), catalyst GP 4590 (3.78%), and a     unneutralized alkyl phosphate ester antistatic agent from E.I. DuPont de      Nemours & Co.                                                                 .sup.2 A polyether diamine based on a polyethylene oxide backbone             terminated in primary amines                                                  .sup.3 Thermoplastic component made from adipic acid and                      diethylenetriamine, CAS No. 2508520-5                                    

                  TABLE 2                                                         ______________________________________                                        Properties of Product Top Spray                                               Property          Value or Description                                        ______________________________________                                        Appearance        Water Clear to Hazy Liquid                                  % Non Volatile    55.00-56.00                                                 Specific Gravity @ 25° C.                                                                1.230-1.240                                                 Viscosity, cps @ 25° C.                                                                  50-150                                                      pH                8.8-9.2                                                     % Free Formaldehyde                                                                             less than about 1.50                                        Water Dilution    greater than about 50:1                                     ______________________________________                                    

Two grams of each liquid resin with 5 grams of water were weighed in analuminum pan and placed in an air circulating oven set at 150° C. for 15minutes. The samples were removed and allowed to cool at roomtemperature. The aluminum pans were flexed several times to observe theflexibility or brittleness of the cured film.

A second set of resin coated aluminum pans were cured at 150° C. for 15minutes, cooled at room temperature, and placed back into the 150° C.oven for 60 seconds. They were removed and quickly checked forflexibility while still hot. By subjective observation it was determinedthat Example 6 was the best among examples 3-7 at providing a hardprotective coating at room temperatures and was sufficiently flexible atembossing temperatures to withstand an embossing process withoutfracturing the board surface. The remaining samples exhibited highlevels of surface cracking where the pattern meets smooth board surface.

Example 8

Table 3 lists the weight ranges for a particularly preferred formulationfor top spray coating according to the invention.

                  TABLE 3                                                         ______________________________________                                        Material        Concentration (%)                                                                           Weight (%)                                      ______________________________________                                        Base MUF Resin GP 5361                                                                        61.0          88-93                                           Catalyst GP 4590                                                                              100.0         1-5                                             Thermoplastic ROPL 2707                                                                       48.0           1-15                                           antistatic agent                                                                              100.0         <1                                              Sodium Hydroxide                                                                              25.0          <1                                              ______________________________________                                    

Example 9

Raw door skin blanks, initially smooth on both sides, are fed into anembossing machine at rates varied to achieve the desired embossing depthwithout burning the board. The blanks travel between a lower stationaryroll and a superheated upper branding die which is hydraulicallycontrolled for predetermined pressures. The upper brand embosses a woodgrain pattern onto the top of the blank.

With prior materials, the tensile strength of the board surface was sohigh that the embossing process at the desired depth caused burning anda fracturing of the surface here the grain ends in a smooth surface. Thematerial of the present invention provides a desirable embossed patternat the desired depth without burning of the surface with elimination ofthe majority of the fracturing.

We claim:
 1. A process for the manufacture of a panel, said processcomprising:forming into a planar panel an admixture comprisingcellulosic fragments coated with a sufficient quantity of binder resinto bind together said fragments under heat and pressure; applying to atleast one surface of said planar panel a top coating exhibiting asoftening point within the range from about 130° to about 300° C.,wherein said top coating comprises: (a) a thermoset resin having freeformaldehyde admixed with said resin; and (b) a thermoplastic polymerthoroughly admixed with said resin, wherein said thermoplastic polymerexhibits amine groups capable of reacting with at least a portion ofsaid free formaldehyde under resin curing conditions; heating the coatedpanel to a temperature and under a pressure sufficient to cure saidbinder resin and said top coating, and embossing a pattern onto thecoated side of said panel.
 2. A process as in claim 1 wherein theapplying step comprises:applying a top coating which comprises: (a) athermoset resin selected from the group consisting of phenoplasts,aminoplasts, and ketone-aldehyde condensation polymers; and (b) athermoplastic polymer selected from the group consisting of polyamidesand aminopolyamides.
 3. A process as in claim 1 wherein the applyingstep comprises:applying a top coating which comprises: (a) a thermosetresin selected from the group consisting of a melamine-urea-formaldehyderesin or a water-soluble, liquid, phenolaldehyde resin; and (b) anaminopolyamide thermoplastic polymer.
 4. A process as in claim 1 furthercomprising the step of allowing the cured, coated panel to cool beforethe embossing step.